Inside the earth, in the deepest portion of the mantle surrounding the planet’s core, there exist two massive patches that resemble large, distinct structures. They appear to be made of materials different from their surroundings, a fact that has fascinated scientists for years. Collectively, these accumulations account for roughly 3 to 9 percent of the planet’s total volume.
The challenge in uncovering the true nature of these formations lies in their extreme depth. They are far beneath the surface, well beyond the reach of any drill. The deepest borehole ever drilled reached about 12,200 meters, a distance that barely scratched the crust and failed to penetrate into the mantle itself.
Earthquakes provide a crucial tool for probing the planet’s depths. Seismic tomography, the method used to image internal structures, pieces together a picture of the planet’s interior. When an earthquake occurs, energy waves radiate outward in all directions. By tracking tremors from many surface locations, scientists construct a three-dimensional map of the inner Earth.
Because rock and liquid densities vary inside the planet, seismic waves travel at different speeds through different materials. This difference in velocity helps geologists infer the type of material the waves traverse and, in turn, reveals contrasts in composition at depth.
One of the patches lies beneath Africa; this region is part of a pair identified as Large Low Shear Velocity Patches, or LLSVPs, where seismic waves slow down as they pass through the surrounding mantle. One such feature, named Tuzo, is estimated to reach about 800 kilometers in height, a staggering measurement equivalent to roughly ninety Mount Everests stacked in a line from base to summit.
Two main hypotheses
Scientists have not yet arrived at a single, definitive explanation for these structures. The evidence points to high density beneath the lower mantle relative to the adjacent mantle, suggesting a material distinct from the surrounding rocks. Seismic tomography shows the presence of these intrusions, but it does not yet reveal their exact composition or precise density. Nonetheless, the consensus is that these regions are made of a different material and are denser than their surroundings.
One widely discussed hypothesis proposes that LLSVPs are vast accumulations of oceanic crust that may have sunk and become buried over billions of years, remaining in place since the early history of the planet. This view imagines ancient crust fragments that gradually settled and were trapped at the base of the mantle.
A second, equally plausible theory suggests that these giant blocks are remnants from a planet that collided with Earth during the dawn of the Solar System. The hypothetical body, named Theia, would have been smaller than Earth—roughly Mars-sized—and the fatal impact would have flung debris into space. Over time, gravity drew these fragments together, ultimately forming the Moon. In this scenario, the lower mantle could contain material derived from Theia, intermingled with Earth’s mantle and preserved in the present day.
Some researchers propose that pieces of Theia’s mantle might still be present within Earth’s mantle, having been incorporated during the collision and remaining there as a dense, mixed layer. In support of this idea, recent simulations conducted in 2021 modeled how such a catastrophic event could leave a lasting imprint on Earth’s interior, suggesting that elements of Theia’s mantle could survive to the present era.
Although the two main explanations differ in origin, both align with the observation that the lower mantle harbors unusually dense material that behaves differently under seismic scrutiny. As technology advances, scientists expect sharper images and more precise measurements that will help distinguish between these competing scenarios and illuminate the true nature of these enigmatic regions.
Reference work: ProGeEarthPlanetSci, 2020, Large Low Shear Velocity Patches and the Deep Mantle. Attribution: ProGeEarthPlanetSci journal article series.
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